(1) Field of the Invention
The present invention relates to a radio receiver and a signal amplifying method in radio receiver and, in particular, to a radio receiver which is capable of receiving the electric waves (radio signals) according to plural types of radio communication modes (systems) and a method of amplifying signals for use in the same.
(2) Description of the Related Art
In the field of the radio communication technique, in particular, the mobile communication technique as in the personal handy phone system, a radio communication system called Frequency Division Multiple Access (FDMA) system has been well known in which the frequency band dealt with by a single base station is divided to be allocated to a plurality of mobile equipment (mobile terminals) so that they may simultaneously make access to the single base station for communication. Incidentally, in this FDMA system, in general, the FM communication adapted system(analog communication system) has been in which the carrier wave is modulated in frequency for transmission.
On the other hand, in recent years, a communication system called Code Division Multiple Access (CDMA) system has been noted. This CDMA system is a digital radio communication system in which a transmitted signal after a primary modulation [desired wave: See FIG. 8(a)] is subjected to a spread spectrum process (secondary modulation) at each mobile terminal by using a different code series to allow the signals (channels) of the same frequency which are transmitted from each mobile terminal to be separated and distinguished from each other, and is also called a Spread Spectrum Multiple Access (SSMA) system because, due to the above-mentioned secondary modulation, the spectrum of the transmitted signal is greatly spread.
This CDMA system (hereinafter referred to simply as “CDMA”) has a disadvantage that, because the frequency band covered by the base station is simultaneously occupied by a plurality of mobile terminals, the utilizing factor of the band is very low. However, on the other hand, since the transmitted signal after the spectrum has been spread is low in power density, it has an advantage that the secret function is extremely high, which has found use in the specific applications such as military communication and the like.
However, in recent years, there has been a trend that this CDMA system is proposed as a communication system for the private mobile communication service, and its service has already been under way in some overseas areas such as USA or Korea. Also in Japan, its service is scheduled to put into operation in the very near future. As described above, if the service utilizing the CDMA system is realized, since its coexistence with the service utilizing other existing service such as the FDMA system is required, it is required also of the mobile terminals that they can receive either signals of CDMA and FDMA (FM) system (dual reception mode) while, at the same time, allowing either signals to be appropriately amplified.
To meet such a demand, in order to achieve such a mobile terminal with dual reception mode (hereinafter referred to as dual mode terminal), as illustrated in, for instance, FIG. 9, in a receiving system (receiving front end) 100, at the side of the output of an amplifier (IF amp) 105 for the intermediate frequency (IF) band, a distribution switch 106 is provided to switch 106 its output according to the communication system (CDMA mode or FM mode) of the received signal so that the output of the IF amplifier 105 may be outputted to one of the CDMA receiving (demodulating) process system 107A and the FM receiving (demodulating) process system 107B.
Incidentally, referring to FIG. 9, reference numeral 101 denotes a low noise signal amplifier (LNA) for amplifying the received radio frequency (RF), 102 a bandpass filter (BPF) for removing the noise signal component of the received RF signal, and 103 a mixer (down-converter) for mixing the received RF signal and a signal from a synthesizer (Phase Locked Loop circuit:PLL) 104 to down-convert the received RF signal to the IF signal.
With such a construction, according to the receiving system 100 of the dual mode terminal shown in FIG. 9, the received RF signal is down-converted to the IF signal by means of the mixer 103, and the IF signal, after amplified by the IF amplifier 105, is outputted to either one of the CDMA receiving process system 107A or FM receiving process system 107B because the output of the distribution switch 106 is switched according to the mode switching signal.
In the CDMA receiving process system 107A, the input IF signal is subjected to the spectrum despreading process and the demodulation process for demodulating a receiving signal of CDMA system while, in the FM receiving process system 107B, the input IF signal is subjected to the frequency demodulation process for demodulating a receiving signal of FM system.
Incidentally, the above-mentioned mode switching signal is generated by CPU's (not shown) detection of whether it is in the CDMA service area (receiving the signal from the base station for CDMA)) or in the FM service area (receiving the signal from the base station for FM).
By the way, with such dual mode terminal, if the CDMA service area and the already existing FM service area overlie geographically and in frequency, in view of the CDMA performance, in order to avoid the interference problem which may be caused by the FM service carrier signals, a high “tertiary input intercept point (IIP3)” is required, during the CDMA mode operation, of the receiving front end 100 (to be concrete, IF amplifier 105).
Here, this IIP3 is a parameter showing a suppression degree of the noise signal (tertiary distortion) occurring to a received signal in a radio communication system of its own (CDMA, for example) due to the tertiary harmonic component of a received signal (interference wave signal) in a different radio communication system. Assuming, for instance, that the IF amplifier 105 exhibits an input/output characteristic (straight line 111 with approximately “1” of gradient) as shown in FIG. 10, it can be represented by the input level (dB) of its intersection 113 (IIP3) with the input/output characteristic of the tertiary distortion indicated by the straight line 112 of approximately “3” of gradient.
Inconsequence, in order to achieve the high IIP3, it will be necessary to shift the intersection 113 (straight line 112) of FIG. 10 in the rightward direction of the sheet, i.e., to reduce the generation of the tertiary distortion on the same input level. Why, during the CDMA mode operation, the high IIP3 is required is that, as described above with reference to FIG. 8(b), in the CDMA system, the power density of the spectrum of the transmitted signal is extremely low as compared with that of the FM system and that if the tertiary distortion caused by the tertiary harmonic wave signal of the FM system carrier signal (interference signal) takes place greatly to the frequency area of this spectrum for interference, then the S/N ratio of the CDMA signal can be greatly degraded resulting its abnormal reception.
Therefore, with the receiving front end 100 shown in FIG. 9, the bias current amount of, for example, the IF amplifier 105 is previously adjusted large so as to adapt itself to the CDMA mode so that the point (saturation point) 114 where the linear input/output characteristic of the IF amplifier 105 begins to deteriorate in the form of a curve is shifted in the rightward direction of the sheet (the linear portion is made wider: See sign 114′) with the result that the performance of the IF amplifier 105 is improved and the above-mentioned generation of the tertiary distortion is suppressed.
As a result, the foregoing IF amplifier 105, whether it is operating in the CDMA or FM mode, may operate with the bias current amount for the CDMA mode which calls for a high IIP3. However, during the FM mode operation, since it does not call for such a high IIP3, if the IF amplifier 105 is used in common with the CDMA and FM modes, then, as compared with the existing receiving front end dedicated to the FM mode, the current amount consumption can become too much resulting in a great increase of power consumption.